1. Field of the Invention
The present invention relates to a digital printer, which prints an image obtained as a digital image data on a recording paper, and relates to a printing method of the digital printer.
2. Description of the Related Art
A digital printer receives a still picture data, which is described a gradation value of each pixel with a digital data, from a host computer or like, hereinafter called a host, and prints the still picture data line by line sequentially. Finally, a sheet of still picture is obtained on a recording paper.
In order to suppress total power capacity of a printer while printing, such a digital printer does not simultaneously conduct all printing elements per line of a printer head and prints with dividing the printing elements into several times. In other words, a number of elements to be simultaneously conducted is limited, so that necessary supply current is limited to not more than a predetermined value. Accordingly, power capacity required for a power supply section of the printer can be suppressed and is advantageous to dimensions and cost of the printer.
FIG. 9 is a block diagram of signal processing circuit in a digital printer performing divisional printing according to the prior art.
FIG. 10 is a timing chart showing behavior of each signal at a gradation number xe2x80x9cLxe2x80x9d according to the prior art.
Various kinds of methods of dividing printing data can be considered. A method of masking picture data transmitted to a printing head by using a data mask signal is depicted hereinafter. Further, a number of divisions is assumed to be 4 as an example.
As shown in FIG. 9, a host not shown retaining a picture data transmits the picture data to a signal processing circuit line by line in order. The picture data of one line is stored in a line memory 1 in accordance with a control signal of a memory controller 2. The memory controller 2 starts on controlling to read out the picture data from the line memory 1 when the picture data of one line is stored completely, and transmits the data to a level comparator 3 pixel by pixel from a head pixel of the line to an end pixel in order. When the last pixel of the line is read out completely, the reading out operation starts once again from a head pixel of the next line data. A line data is read out repeatedly as many times as a number of printing gradations.
A gradation data from a level counter 21 is inputted to the level comparator 3 in conjunction with a pixel data read out from the line memory 1. The level counter 21 starts on counting from a count number xe2x80x9c0xe2x80x9d and counts up one by one at each time when a count finish signal is inputted from a division number counter 24 in conjunction with starting on reading out a line data from the line memory 1. The level counter 21 keeps counting up until a counting value reaches a value of printing gradation number.
The level comparator 3 compares a picture data from the line memory 1 with a level count value. In a case that a picture data value is larger than a level count value, the level comparator 3 outputs xe2x80x9c1xe2x80x9d (High level signal) and the pixel becomes a printing pixel at a gradation number indicated by a current level count value. In a case that a picture data value is equal to or smaller than a level count value, the level comparator outputs xe2x80x9c0xe2x80x9d (Low level signal) and the pixel is not a printing pixel. The output from the level comparator 3 is transmitted to a thermal head not shown through an AND gate 13 and becomes a printing data.
A count number in the division number counter 24 is reset to"" xe2x80x9c0xe2x80x9d at each time when each gradation printing starts. The count number is counted up one by one at each period of reading out line data. Printing of the current gradation finishes when the count number is counted up from xe2x80x9c0xe2x80x9d to xe2x80x9c3xe2x80x9d, and the count number is reset to xe2x80x9c0xe2x80x9d when a next gradation printing starts. The count number is inputted to a data mask selecting section 23. The division number counter 24 transmits a signal of count finish to the level counter 21 at each time when counting is completed and printing is shifted to a next gradation, and make the level counter 21 count up.
A data mask producing section 22 produces 4 data mask signals (data masks xe2x80x9c0xe2x80x9d through xe2x80x9c3xe2x80x9d), which divide the period of reading out line data into 4 equally such as data masks xe2x80x9c0xe2x80x9d through xe2x80x9c3xe2x80x9d shown in FIG. 10. Each data mask is inputted into the data mask selecting section 23 and one data mask is selected in accordance with a count value outputted from the division number counter 24. The selected data mask signal is inputted into the AND gate 13 in conjunction with a pixel data from the level comparator 3. FIG. 10 shows a case that the data mask xe2x80x9c0xe2x80x9d is selected in a first line data readout period.
Accordingly, a line printing is performed 4 times per each gradation. Since a different data mask is selected at each time and a picture data from the level comparator 3 is masked in the AND gate 13, printing pixels constituting a line are divided into a plurality of pairs by a time sharing method at each gradation and transmitted to the thermal head not shown in FIG. 9.
When a data of a first line is completely read out throughout all printing gradations, a data of one line is transferred to the thermal head completely. The memory controller 2 writes data of a second line from the host into the line memory 1 and controls on reading out the data of the second line from the line memory 1. The signal processing circuit shown in FIG. 9 keeps transferring a picture data from the host to the thermal head through the above mentioned signal processing. Processing picture data of one field comes to end when data of all lines are completely processed such that data of a third line, fourth line and up to a final line are repeatedly read out and wrote in according to an order of line. In a case of color printing, a color picture is obtained by printing the three primary colors of yellow, magenta and cyan repeatedly 3 times for 3 fields.
FIG. 11 shows an electrical configuration of a thermal head xe2x80x9cSHxe2x80x9d according to the prior art. The thermal head xe2x80x9cSHxe2x80x9d comprises two groups of registers, which equal to a number of line dots multiplied by 1 bit. Each register RE of a first register group 31 is corresponding to each heat generating element 35 and decides to turn on or off electricity to the heat generating element 35 by hold parameters of the register RE. The thermal head xe2x80x9cSHxe2x80x9d receives a serial picture data of 1 bit wide from the AND gate 13 shown in FIG. 9 and the serial picture data is sequentially accumulated in the first register group 31 constituting a shift register. When accumulation of data of one line is completed, data of the first register group 31 are transferred to a second register group 32 by a register set signal emitted from a thermal head controlling section not shown. The heat generating element 35 is turned on in response to a data value of the second register group 32 and prints a first time divisional printing. A data of next one line is written in the first register group 31 at a same time as printing the first time divisional printing. A turn-on period is controlled by a pulse width of a control signal of turn-on pulse width emitted from the thermal head controlling section not shown. A signal performed by the AND operation between a truth or falsehood value of each register in the second register group 32 and a control signal of turn-on pulse width at each AND gate 33 is inputted to a turn-on switch element 34. In a case that an input value of the signal is truth, the heat generating element 35 is turned on.
According to the controlling method mentioned above, a division number is fixed. However, the Japanese Patent Laid-open Publication No. 62-58584 discloses the controlling method of varying a number of sections to be simultaneously turned on in response to a total number of printing pixels after dividing a line into plural sections. According to the controlling method, a division number is dynamically changed, so that it is advantageous for a printer to print at a higher rate of printing speed comparing with the printing method of always fixing the division number to a predetermined value as mentioned above.
According to the divisional printing method of the prior art mentioned above, printing pixels of one line are divided into a plurality of data and each data is printed with staggering the time. Therefore, a power capacity can be suppressed by limiting a number of simultaneous turn-on dots and suppressing power consumption per unit hour. However, a number of printing dots per each gradation is not perfectly maintained in a predetermined value although a number of simultaneous turn-on pixels can be limited to less than the predetermined value. The number of simultaneous turn-on pixels varies by contents of a picture data. Accordingly, a power supply for driving thermal head is affected by a load fluctuation caused by change of a number of simultaneous turn-on pixels, so that an output voltage varies. The voltage variation affects printing density and creates a problem of deteriorating picture quality.
Further, the controlling method of the Japanese Patent Laid-open Publication No. 62-58584 discloses the effects of increasing the printing speed comparing with the printing method of always fixing the division number to a predetermined value as well as suppressing power consumption. However, a number of turn-on pixels can only be controlled by a unit of divisional section, so that scattering of a number of simultaneous turn-on pixels occurs. Accordingly, the controlling method does not improve efficiency for a higher printing speed.
Accordingly, in consideration of the above-mentioned problem of the prior art, an object of the present invention is to provide a digital printer, which always maintains a number of simultaneous turn-on pixels except a special case.
In order to achieve the above object, the present invention provides a digital printer, which receives a digital picture data composed of pixels having a multi-gradation data respectively and obtains a line composed of printing pixels having a density difference of multi-gradations on a recording paper by controlling the digital picture data with a line head whether or not there exist ink transcription at each gradation and prints a picture by recording a plurality of lines obtained by scanning with the line head on a recording paper, the digital printer comprising counting means for counting a number of pixels for printing at each gradation, dividing means for dividing total pixels into several groups in accordance with a count value of said counting means and deciding means for deciding a dividing number of groups, the digital printer is further characterized by that a range of grouping and a dividing number of groups are changeable so as for a number of printing pixels in the groups to be equal to a predetermined number as a number of simultaneous printing pixels, and that a pixel of simultaneously printing in one line is limited to a unit of the group during printing operation at each gradation, and that printing of each gradation is divided into a plurality of times.
According to an aspect of the present invention, there provided a printing method of a digital printer, which receives a digital picture data composed of pixels having a multi-gradation data respectively and obtains a line composed of printing pixels having a density difference of multi-gradations on a recording paper by controlling the digital picture data with a line head whether or not there exist ink transcription at each gradation and prints a picture by recording a plurality of lines obtained by scanning with the line head on a recording paper, the printing method comprising a step of counting a number of pixels for printing at each gradation, a step of dividing total pixels into several groups in accordance with a count value of the counting means and a step of deciding a dividing number of groups, the printing method is further characterized by that a range of grouping and a dividing number of groups are changeable so as for a number of printing pixels in the groups to be equal to a predetermined number as a number of simultaneous printing pixels, and that a pixel of simultaneously printing in one line is limited to a unit of the group during printing operation at each gradation, and that printing of each gradation is divided into a plurality of times.